This invention relates to horizontal deflection amplifiers for resonant scan raster television displays. More specifically it relates to horizontal deflection output circuits which operate with horizontal drive inputs of various frequencies and also contain circuitry to protect the horizontal output transistor from certain types of failures.
It is common to drive the horizontal output transistor of a horizontal deflection circuit with a transformer as disclosed in U.S. Pat. No. 3,434,005 granted to E. J. Dreiske, et al. It is also known to drive the horizontal output transistor base with known currents; i.e., turn on current is determined by its beta requirements and turn off current is approximately equal to one half of the peak collector current. Separately establishing these currents assures a high efficiency transistor switch. This drive method was first discussed in the article "Horizontal Output Transistor Base Circuit Design" by R. J. Walker and R. Yu in IEEE Transactions on Broadcast and Television Receivers, Vol. BTR-20, No. 3, August 1974, pp. 185-192. However, a method of accurately establishing this current is not disclosed.
Various prior art techniques for protecting horizontal deflection circuits against various fault modes are known. These techniques generally protect against power supply and horizontal deflection faults as well as other faults which produce an effect in the power supply or horizontal deflection circuit. U.S. Pat. No. 4,042,858 granted to Collette, et al., for example, discloses a protection circuit which detects excessive current drain by a low voltage supply, excessive current drain by a high voltage, and excessive high voltage. If any one of these conditions is detected, the horizontal output transistor is disabled. Also, the previously mentioned patent granted to Dreiske, et al. discloses a circuit for protecting the output transistor against permanent damage that may occur because of failure of the high-voltage rectifiers.
Another failure mode which occurs in horizontal deflection systems and may cause permanent damage to the horizontal output transistor is failure of the horizontal oscillator or the horizontal drive transistor. Permanent damage may also occur if the output frequency of the horizontal oscillator is suddenly decreased. In the circuit described in the Dreiske, et al., for example, if the driver stage transistor is suddenly turned off for a long time, the voltage induced in the secondary winding of the horizontal output stage coupling transformer may cause the next flyback pulse to exceed the breakdown voltage of the horizontal output transistor. Conversely, if the horizontal driver transistor is suddenly turned on for a long time, the driver stage transistor and coupling transformer may be damaged.